Organic light-emitting diodes, which when abbreviated are also and usually designated as OLEDs, have in particular in an embodiment for white light generation a high potential for applications in the field of illumination and displays. Substantial improvements have been achieved in this field in recent years, not only with regard to the efficiencies obtained but also with regard to the life service duration of the devices. The performance efficiencies of white OLEDs are presently in the range of 10 to 25 lm/W, and life service durations of over 10000 hours are realizable. For a wide-scale commercialization in the field of general illumination applications, however, improvements with particular reference to performance efficiency are necessary because at the moment the market is controlled by high-efficient technologies for the generation of white light, such as fluorescent tubes for example, with efficiencies of up to 100 lm/W.
It is generally known that conventional OLEDs only emit about 25% of the generated light, while the major part remains in optical modes of the substrate or the organic layers, and is subsequently lost in the process. The reason here is that the light within an OLED in optical media is formed with a refractive index of approx. 1.3 to 1.8. If this light now impinges on an optical medium with a low refractive index, for example a further layer within an OLED-stack, the substrate on which the OLED is established, or one of the electrodes, then a total reflection occurs when a certain value of the angle of incidence is exceeded.
Particularly during the transition of the light into air, such a refractive index leap necessarily occurs, through which merely light, which impinges onto the boundary surface to air under relatively small angles, can emerge from the OLED device. This boundary angle for the total reflection amounts to arcsin(1/n) during transition to air, where n is the refractive index of the material on the boundary line to air.
Manifold considerations are known, which are focused on an improvement of this light extraction. Worth mentioning would be, for example, the depositing of scatter foils, microlenses or specially structured surfaces on a substrate side of the OLED, which forms the boundary surface towards the air and where a major part of the undesirable total reflection of the light takes place. These measures are aimed at extracting light, which “is captured” in the transparent substrates of the OLEDs.
Structural elements on the basis of OLEDs, however, which are deposited on a reflecting base contact, so-called top-emitting OLEDs, are not limited in their performance capacity by losses in the transparent substrate. Nevertheless, in this case also, the luminous efficiency is restricted in principle by the loss mechanisms because a major part of the emitted light “is captured” now within the organic layers of the OLED due to total reflection. Possible improvements of the luminous efficiency are known here also, particularly by means of the formation of so-called micro cavities or also with the help of additional extraction layers, which improve the reflection conditions at the boundary surface of the OLED in the direction towards air.
Despite the already achieved improvements for extraction of the emitted light, considerable improvements are still necessary compared with the state of the art. This particularly involves light which itself is captured in the modes of the organic layers of the OLED. It is known that the light captured here is usable in principle as soon as the organic layers are not located on a smooth substrate but when this surface is grooved, for example. As a result, light hits the boundary surface of the area of organic layers at various angles and can be extracted in this way, as shown schematically in FIGS. 1A and 1B. This description by means of geometrical radiation optics is only approximately correct with regard to layer thicknesses which can lie below the light wavelength, but it describes the phenomenology of non-smooth substrates to a sufficient degree. Such methods have already been described, for example in the document GB 2361356 A for transparent substrates with transparent bottom contact electrode or in the document GB 2390215 A for active matrix display elements with grooved pixel structure.
For the use of white OLEDs in the illumination technology it is therefore necessary to adopt suitable extraction methods which can, in addition to this, be incorporated inexpensively into the manufacturing process. At present, it is taken for granted that an OLED surface of 1 cm2 for illumination purposes shall cost only a few cents so that its usage is economically purposeful. However, this also means that only particularly inexpensive methods can be selectable in any event for the increase of light extraction.
For applications of OLEDs as illuminating elements it is furthermore necessary to execute the devices on a large-surface scale. If, for example, an OLED is operated with a luminance of 1000 cd/m2, surfaces in the region of some square meters will be required in order to illuminate an office room, for example.
With the construction of OLEDs of this size, however, there is a problem with the electrode conductivity of the transparent electrode of the device. Normally, OLEDs are deposited on transparent substrates coated with ITO (indium tin oxide) where ITO, however and depending on the layer thickness and composition, has a layer resistance of between 5 and 100 Ω/∀. For large-surface OLEDs, however, significantly smaller layer resistances are required because relatively high currents have to be transported through the electrodes and, thus, a significant voltage drop can occur across the electrodes, insofar as these have very small resistances smaller than 1 Ω/∀, for example. Due to the layer resistance of the ITO, with OLED surfaces of even a few square centimeters, this effect leads to significant luminance decays. In order to reduce this problem, the ITO can be reinforced with additional metal tracks, which then transport the major part of the flowing currents. For this purpose, however, the metal tracks must have a certain diameter, for which reason they normally have a height of some hundred nanometers as they would be otherwise too wide, and through which the active surface of the OLED structural element is too strongly reduced. With the use of transparent ITO-coated substrates with a metal reinforcement, however, a passivation of the metal tracks with an insulating layer is necessary because short-circuits can otherwise occur between the bottom and the top electrode of the device.
Taking these facts into account, the use of highly conductive bottom contacts on the substrate would seem purposeful. Electrodes with a correspondingly high conductivity of <1 Ω/∀ can only be obtained with metals according to the state of the art. Corresponding conductivities can, however, only be achieved with layer thicknesses which then only have a very low level transparency or are not transparent at all. Therefore, with the use of highly conductive bottom contacts, the OLED must have a transparent top electrode, so that it must be executed in a top-emitting manner. Such a transparent top electrode can, for example, consist of thin metal layers or ITO.
The considerations with regard to layer resistance apply also on the same scale for the top electrode also. The conductivity of the transparent electrode is thus also inadequate here in order to transport the current flowing in the OLED without significant voltage losses, insofar as the device exceeds a certain size as would seem necessary for illumination applications. However, a reinforcement of the top electrode by means of metal conductor tracks is achievable to a significantly easier degree in this case because an additional passivation step can now be dispensed with. The metallic reinforcements for the current transport can in fact be deposited onto the top electrode in the form of a lattice, for example. The danger of a shortcircuit between anode and cathode of the OLED does no longer have to be taken into consideration.
In this case, the processing of the metal tracks is relatively uncomplicated as only structures have to be formed in dimensions, which enable a current flow through the electrode without any significant voltage drop. For example, the metal tracks could form a grid with a mesh size of approximately one centimeter, through which the voltage drop can be minimized to a negligible value by means of the segmented and transparent electrode partial surfaces established in this way.
Of course, there are also alternative approaches for bypassing the problem, which results on the basis of a serial resistance during the transport of current. Particularly worth mentioning here is the fact that, repeatedly, the series connection of OLED units within a device was proposed. However, such a solution presupposes a more complex structure of the substrate as well as additional masking steps.
Substantial progress with regard to efficiency as well as life duration service has been achieved in recent times for top-emitting OLEDs. For this reason, it can be assumed that top-emitting structures are of major interest in the field of illumination applications, among other things also because of the option of dispensing with ITO as a material, which is a significant cost factor for OLEDs. For an economical breakthrough of the OLED technology, however, and as already mentioned, an inexpensive production with simultaneous high efficiency of the device is necessary in particular. For this purpose, for example, attention is focused on production methods in a “roll-to-roll” process. In order to develop an economically competitive OLED illumination technology, it is furthermore particularly necessary to inexpensively establish any possible extraction methods.
Up to the present, it was not possible to commercialize the OLED technology for applications as illuminating elements, particularly in view of the fact that efficiencies and price of OLEDs are not yet competitive. Methods are described in which scatter lattices (refer to U.S. Pat. No. 6,476,550), two-dimensional photonic structures (refer to U.S. Pat. No. 6,630,684), holograms or the like are integrated for the purpose of extraction improvement. However, these methods are based on relatively complex process steps, photolithography for example.
For so-called bottom-emitting OLED structures in which a transparent substrate is used, through which the light emission is effected all the way through and in the downward direction, it was proposed to apply regular structures by means of a stamp (refer to GB 2361356 A). Methods are described in this document for the manufacture of a grooved surface on a transparent substrate where polymer layers are formed with the use of a tool. In this case, methods are disclosed in which the form-shaping is effected by means of photo-hardening of a liquid polymer, which is kept in shape during the hardening process with the help of a form tool, or with which a polymer solution is formed with a tool and hardened by means of the evaporation of the solvent. For top-emitting OLED structures, however, structured bottom electrodes are deployed according to the state of the art by means of complex methods (refer to GB 2390215 A), for example photolithography, and these bottom electrodes are not selectable for economical mass production. One reason for this is the fact that it is assumed for top-emitting OLEDs in the state of the art that the surfaces of the substrates that are used have to be very smooth as short-circuits occur on rough surfaces. In the case of the bottom-emitting devices a completely different bottom electrode is envisaged that is usually made from a conductive glass, whereas typical bottom electrodes for top-emitting devices normally consist of a metal or a metal stack. This difference at the boundary surface between substrate and the zone of organic layers means that there are very different requirements for top-emitting and bottom-emitting devices with regard to the architecture of the zone of organic layers and of the substrate. Therefore, only a few highly efficient and long-life top-emitting devices are known whereas, on the other hand, a plurality of bottom-emitting devices has been described.
In particular the roughness of the bottom electrode is a problem for a processing of OLEDs because very thin overall layer thicknesses of approximately 100 nm are normally used for the zone of the organic layers in the OLED stack, through which there is a danger of short-circuiting between the electrodes. For this reason, either special planarizing layers have to be deployed in active matrix display elements, for example, or the bottom electrode surface must be executed in such a way that existing height differences are provided with suitably flat edge angles, through which an inclination to short circuiting can be suppressed.